This invention relates to internal combustion engine operation and control, and more specifically, to engine operation and control responsive to fuel volatility.
The need to be able to effectively start and run an internal combustion (IC) engine using fuels with a range of properties has been a problem that continually challenges engine calibrators. The fuel properties that pose problems include the vaporization pressure of the fuel, which is quantified by the Reid Vapor Pressure (RVP) or the Driveability Index (DI). Fuel refiners and distributors adjust the fuel vaporization pressure to correspond to seasonal ambient temperatures in order to optimize the cold start capability of IC engines in various geographic regions. This variation in vaporization pressure is created by balancing the amount of lower-, mid-, and heavier-weight hydrocarbon molecules in the fuel. The lower weight hydrocarbon molecules vaporize at lower temperatures, thus leading to more effective engine startability at low ambient temperatures. The fuel available can range in DI from under 1000 (highly volatile) in cooler areas to over 1250 (very stable) in hotter areas.
In addition, the fuel in a fuel tank may change vaporization characteristics over time, through a process called xe2x80x98weatheringxe2x80x99. The lower-weight hydrocarbon molecules may evaporate in the fuel tank. Passenger cars and trucks have evaporative systems that capture and store these evaporated hydrocarbons in a carbon canister and subsequently consume them by purging the canister through the engine. In engine applications where there is no evaporative system, these lower weight molecules may be vented to the atmosphere. Either way, the evaporative characteristics of the fuel remaining will have changed, and the suitability of the fuel for cold start operation will have also changed.
Engine manufacturers are faced with meeting requirements for stable start and run conditions. To meet the driveability requirements, engine management systems are calibrated using a sufficient amount of fuel to be robust when fuels of varying volatility are encountered. A typical approach to managing varying levels of fuel volatility has been to calibrate the system with excess fuel to ensure good driveability during engine start and initial operation. This use of excess fuel increases engine-out hydrocarbon and carbon monoxide emissions. In addition, the vehicle manufacturers must also comply with more stringent exhaust emissions regulations. An important strategy in meeting these emission regulations is to ensure that the engine runs at an air/fuel (AF) ratio that is at or near stoichiometry at the start of the engine, or soon thereafter. This is necessary to minimize engine out emissions and also to provide an exhaust gas feedstream to the catalytic converter that allows the converter to perform at optimum levels.
Engine and vehicle manufacturers accomplish this balance between meeting requirements for stable operation and meeting emissions regulations several ways. Extensive testing and calibration during the engine development phase is conducted. Hardware such as air injection pumps are added. The amount of precious metals (Palladium, Rhodium, and Platinum) contained in the catalytic converter is increased to improve effective conversion of pollutants. Each of these methods adds complexity and cost to the vehicle or engine.
Optimal operation and control of an engine occurs when the engine is in a warmed up state and is using an exhaust gas sensor to provide feedback to the engine controller for closed loop control of the engine. During the initial operation of the engine after start, especially a cold start, an engine may not be able to operate in closed loop fashion based on feedback from the exhaust gas sensor. This may happen for several reasons. The exhaust gas sensor typically takes a certain amount of time to become functional, i.e. to warm up or xe2x80x98light-offxe2x80x99. This sensor light-off may take a few seconds, or it may take more than 30 seconds, depending on sensor design and placement, ambient conditions, and the temperature of the exhaust gas from the engine.
In addition to the exhaust gas sensor not being functional, the engine itself may not be sufficiently warmed up to operate at or near stoichiometry immediately after start. An engine that has high internal friction may require that there be more power to operate the engine when it is cold. This can lead to a need for corresponding rich AF operation to overcome the friction. Another factor is the design of the intake manifold, including placement of the fuel injectors. This can affect the amount of fuel that must be delivered to have a sufficient quantity of fuel vaporized to effectively operate the engine. Also, the design of the exhaust catalytic converter system may require the engine to operate in a manner that enables the catalyst to quickly light off and become chemically active. Balanced against this is the need to provide smooth, stable engine operation and the need to minimize tailpipe emissions.
Prior to sensor light-off the engine controller relies upon information other than the input from the exhaust gas sensor to control the engine. This includes inputs from other sensors and calibrations that are internal to the controller, e.g. crankshaft sensor, manifold absolute pressure sensor, and throttle position sensor. The engine controller can control the engine to a commanded AF ratio by monitoring input from the sensors and by using internal calibrations. This control will be based on the engine operating conditions that can be measured directly and conditions that are inferred from predictable behavior of the engine under the measured conditions.
The engine controller is still unable to manage engine roughness resulting from incomplete vaporization of the fuel and mixture with the air in individual cylinders that can occur as the result of unanticipated fuel volatility. This engine roughness will be manifested as instability in the engine crank speed during initial engine operation. Variations in crankshaft speed can also be related to design of the engine and variation in components of each cylinder due to part-to-part variability, deterioration of engine components and sensors, or various component or system malfunctions.
The prior art has sought to measure and compensate for fuel volatility by monitoring engine performance during initial engine operation. This is accomplished by monitoring the engine speed immediately after starting and comparing it to predetermined engine speed values that have been determined by testing prototype engines. This type of method will provide fuel compensation during initial operation based only on the measured engine speed. However, it is recognized that variations in fuel volatility affect the initial operation of the engine in several ways beyond the initial engine run speed. The prior art does not provide compensation for fuel volatility during engine cranking. Nor does it compensate for other effects of variation in fuel volatility, including engine roughness, and instability in engine firing during initial operation. The prior art also does not address a root cause of varying engine performance as a result varying fuel volatility, which is the effect of intake valve temperature on the incoming fuel.
Accordingly, a need exists for a more complete method to compensate for the variation in fuel volatility during engine start and initial operation.
The present invention provides an improvement over conventional engine controls by adjusting engine air/fuel [AF] ratio, as a function of fuel volatility, during engine start and initial operation. The AF ratio is used in the calculation of the engine fueling. Engine fueling is calculated by determining the air intake to the engine and dividing this value by the AF ratio.
The present invention is a method that is comprised of three detection tests to determine the volatility of the fuel and provide compensation to the AF ratio in proportion to that volatility during engine crank and initial operation. In the first detection test, crank time is monitored and compared to a threshold, which is a function of coolant temperature. If the crank time exceeds the threshold, an offset, called a crank AF adjustment term (CAFAT), is calculated based on the amount of time the threshold crank time is exceeded. This test ends when the engine makes the transition from start to run, and the crank AF adjustment term is stored for use during subsequent engine starting events. During subsequent engine starts, the crank AF adjustment term will be added to a base crank AF ratio to create a crank AF ratio which will be used by the engine controller to start the engine.
In the second detection test, the engine speed is measured at a specific time after the engine has been started, and is then compared to an engine speed threshold. If the engine speed falls below the threshold, a second term, called a first AF compensation term, is calculated that is based on the ratio of the engine speed and the engine speed threshold. This test occurs once per engine start and run cycle.
In the third detection test, engine roughness during initial operation of the engine is monitored. The engine roughness is compared to a threshold, and a second AF compensation term is calculated based on the magnitude of the engine roughness. This test can end after a specific amount of time, or a specific number of engine revolutions, or after a specific event has occurred, such as the engine going into closed loop operation. In an alternate embodiment, the test will estimate a temperature of an intake valve during the initial operation, and calculate the second AF compensation term based upon the magnitude of the engine roughness measured prior to when the estimate the temperature of an intake valve exceeds a threshold value.
A run AF adjustment term is then selected by comparing the first AF compensation term and the second AF compensation term, and selecting the greater value. The greater value will become the run AF adjustment term and can be stored for use on subsequent engine start and run events. During subsequent engine start and run events, the run AF adjustment term will be added to a base run AF ratio to create a run AF ratio which will be used by the engine controller to run the engine during initial operation.
It is an object of this invention to provide compensation for varying fuel volatility during engine crank and initial operation, as manifested by engine crank time, engine operating speed until a given amount of time, and engine roughness during the initial operation.
It is a further object of this invention to provide compensation for varying fuel volatility during initial operation, by compensating for incomplete fuel vaporization as a function of predicted intake valve temperature.
These and other objects of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description of the embodiments.